I think this began when the auto-update to 22 occurred. I run Firefox on a PC with Windows 7 AND a Macbook Pro running 10.8.3 and the same error is occurring on both machines. At an increasing rate, as if some cache is filling up, I get errors that say a script is slow or not responding and identifying it as various javascripts. Eventually--and always if I write a long email, it's like working in marmalade and I knock everything down, reset and try again. I use Icloud as my primary email program on both machines. I have deleted java, installed new 32 and 64 bit java and even gone to 64-bit Waterfox but nothing has solved the problem. I've installed, disabled and re-installed every damn extension. Now, when 18 first went in, I lost the ability to see any javascript based emails -- e.g. from Wordpress--unless I forwarded them to myself. This suddenly fixed itself so I imagine people are trying to catch up with bugs caused by the update but this slow javascript thing is NOT getting any better.